The present invention relates to steam turbine rotors, and more particularly to dovetail connections between steam turbine rotor wheels and steam turbine buckets.
Dovetail connections between turbine rotor wheels and turbine buckets include wheel hooks and bucket hooks that cooperate with each other to counter the centrifugal forces that are imposed on the connections. These hooks also prevent the buckets from separating from their wheel during the operation of the steam turbine. However, a major failure potential for conventional dovetail connections is their margin for creep. This is especially true in high pressure (HP) sections of the turbine where high temperatures are produced. For a bucket and wheel dovetail connection in a turbine operating at operational temperature of between about 850 to 1000 degrees Fahrenheit and at given stress levels, the creep strength of the bucket and rotor materials is not equal. Instead, the strength of the wheel is less than that of the bucket. As a consequence of the material strength differences and the load distribution that occurs during operation, the lower strength wheel limits the magnitude of the stresses that the connection can withstand. If the stresses exceed the material strength of the wheel, creeping will occur and the connection may fail. The limitations created by the configuration and dimensions of these conventional connections and the dependence of these connections on the lower material strength of the wheel prevent conventional turbines from reaching optimal levels of performance.
Commonly assigned U.S. Pat. No. 5,474,423 to Seeley et al. discloses a bucket and wheel dovetail connection for steam turbine rotors. In the Seeley et al. patent, the dovetail connection includes four hooks on the rotor wheel that have crush surfaces. The crush surfaces each form an angle with a respective neck surface that is greater than ninety degrees. The radially innermost hook includes a crush surface that is at an angle of ninety degrees to its respective neck surface. The slanted crush surfaces result in lower stress concentrations in the wheel hooks. Nevertheless, the need for optimization of a dovetail connection between a wheel and a bucket still existed.
These conventional dovetail connections cannot support a larger vane unless the size of the connection is also increased. Applying a larger vane would cause a turbine using conventional dovetail connections to fail. Alternatively, an increase in the overall size of the dovetail connections would result in an undesirable increase in the overall size of the turbine stages and an increase in the spacing of the turbine stages. These increases would require that the overall size of the steam turbine also be increased.
It is desirable to overcome the drawbacks in the prior art by providing a dovetail connection that optimizes the joint between a rotor wheel and a bucket and permits the use of wider vanes at the ends of the buckets without changing the size of the wheel and the other existing components of the turbine. In an embodiment of the present invention, a dovetail connection for connecting a rotor wheel and bucket for a turbine comprises a male dovetail component and a female dovetail component. The male dovetail component includes first and second hooks. Each of the hooks includes a crush surface, a neck and an angle formed between the crush surface and the neck. These hooks are dimensioned in accordance with at least one of the below listed tables.